This project (1) examines cancer incidence and mortality among populations exposed to ionizing radiation, especially at low dose levels; (2) characterizes the risk of radiation-induced cancer in terms of tissues at risk, dose response, radiation quality, fractionation of dose, time since exposure, sex, age at exposure and at observation, and possible modifying influences of other environmental and host factors; and (3) examines, tests, and formulates models of radiation carcinogenesis to help define basic mechanisms. Groups studied include the Japanese A-bomb survivors, and several large populations with documented therapeutic (e.g., cervical cancer patients), diagnostic (e.g., tuberculosis patients), and occupational (e.g., x-ray technologists) exposures to ionizing radiation. Program members serve on committees advising the government as well as international agencies. Results of studies suggest that (1) susceptibility to radiogenic breast cancer declines with increasing age at exposure, and children exposed under age 10 are at high risk; (2) repeated exposure to relatively low radiation doses pose some future risk of breast cancer; (3) the risk of radiogenic cancer may remain throughout life; (4) prenatal x-ray in twins increases the risk of childhood cancer; (5) children irradiated for benign conditions of the head and neck are at high risk of developing thyroid and brain neoplasia; (6) 9% of all thyroid cancers may be attributed to prior childhood head and neck irradiation; (7) radiotherpy for childhood cancer was associated with increased risks of subsequent bone cancer but not leukemia; (8) high-dose radiation to the pelvis induces fewer leukemias than other types of exposures; (9) ovarian damage caused by radiation may lower breask cancer risk, even among postmenopausal women; (10) chromosome aberrations following partial-body irradiation persist in circulating lymphocytes for over 30 years; (11) radiogenic colon cancer occurs in atomic bomb survivors but not in cervical cancer patients treated with radition.